In the fabrication of integrated circuitry, increased miniaturization of electronic devices is an important goal. In the fabrication of thin film integrated optic components, such as light guides, modulators, directional couplers, and polarizers, high resolution pattern formation of submicrometer size is required, as well as high quality edge smoothness of the pattern. In order to decrease the size of the devices, one key step in the fabrication of these small geometry devices such as field-effect transistors, for example, is the accurate and precise definition of the patterns used to define gate, source, or drain regions or to define metal contact areas thereto. Using known processes, a photoresist pattern is defined on the surface of a substrate or a layer formed thereon, as described by William S. DeForest in the book entitled, "Photoresist Materials and Processes," McGraw-Hill Book Company, New York, 1975. Then, a chemical etchant which is specific for the substance to be removed is applied through openings in the photoresist pattern to etch selected regions of the substrate or layer formed on the substrate. Chemical etching has, however, several disadvantages, one of which is to cause delamination of the resist by attacking the resist-substrate interface and thus causing a loss of edge definition of the resist pattern and less precise definition of the etched pattern. In some cases, the etchant required for the material to be removed would chemically attack and destroy the photoresist material. In addition, in chemical etching, lateral etching of the substrate, or undercutting, can occur, in which portions of the substrate which it was desired to maintain are etched away, thus interfering with the formation of the desired structure.
In order to alleviate some of the problems associated with the use of chemical etchants, an energetically controlled beam of ions has been used to etch a surface through a patterned mask, as disclosed in U.S. Pat. Nos. 3,860,783, and further in 3,988,564, the latter being assigned to the present assignee. The patterns produced by ion beam etching are more sharply defined than those produced by chemical etching and the quality of the pattern definition and therefore the performance of the resulting device are improved. Ion beam etching is the result of a transfer of momentum between the incident ions and the target atoms, in which the surface atoms of the target are imparted with sufficient energy to allow them to escape from the surface. For given conditions of incident ion species and bombardment energy, different materials are etched at different rates, which have been related, at least in part, to atomic numbers. (See N. Laegreid and G. K. Wehner, Journal of Applied Physics, Vol. 32, p. 365 (1961).) The formation of stable compounds, such as oxides or nitrides, on the surface of the target has been found to reduce the ion beam etching rate at these surfaces. When a target is etched by an ion beam which is applied through openings in a resist mask to the target, molecules of the resist as well as molecules of the target are etched. Thus, it is necessary in order to maintain the desired pattern, to provide an initial resist layer of sufficient thickness that some resist will remain when the target material has been etched to the desired depth. As the resist is etched during the ion beam etching process, however, the sharpness of the resist pattern deteriorates, i.e., the edge profile of the openings in the resist is not maintained. In addition, the deeper the substrate is etched, the more severe is the erosion of the resist pattern.
This problem of resist erosion is of particular importance where the resist used as the ion beam etching mask remains in place to further serve as a mask for metal deposition to establish metal contacts to the device. The resist profile is of primary importance to the function of defining metal patterns with respect to edge quality and the thickness attainable. When the resist pattern becomes eroded during the course of a prior art ion beam etching process, the edges of the resist pattern at the top surface take the form of gradually sloped walls, rather than a sharp edge approximating 90.degree.. When metal is subsequently deposited over this photoresist, the metal conforms to the shape of photoresist layer and forms a continuous layer over the top surface of the photoresist, the sloping walls of the photoresist which lie on either side of the etched region of the target, and the bottom surface of the etched region of the target. A metal layer of this configuration has structural strength and uniformity of coverage which cause difficulty during lift-off procedures when the resist is dissolved and the unwanted metal is removed with the resist. In some cases, the undesired metal must actually be torn away from the desired metal. Naturally, the thicker the layer of metal which is deposited, the greater the strength of the layer and the more severe the problem at lift-off. Thus, by prior art processes, the thickness of the deposited metal layer is limited.